1. Field of the Invention
The present invention relates to an optical encoder and a scale for an encoder, and in particular, to an optical encoder which detects an amount of movement of a scale by utilizing changes in a diffraction interference pattern accompanying movement of the scale, and to a scale for an encoder which is used in the optical encoder.
2. Description of the Related Art
In recent years, as semiconductor devices and optical devices have become more precise, displacement detecting systems which have high resolution and are highly precise, such as laser interferometric measuring machines and the like, have come to be used. Most recently, attention has focused on an extremely high precision optical linear encoder using a hologram scale (hereinafter called xe2x80x9chologram encoderxe2x80x9d) as a high-performance displacement detecting system which rivals laser interferometric measuring machines.
At a hologram scale, the wave front of light is directly recorded on a graduation surface in the form of a hologram. In light of the principles of the method of manufacturing thereof, the finest graduation pitch which is of the order of the wavelength of the light can be realized. A hologram encoder detects displacement by using the interference of the diffracted light caused by the hologram recorded on the scale, and has the highest resolution and precision among various types of linear encoders.
As will be described hereinafter, the hologram encoder utilizes the interference of diffracted light in the principles of detection, and the hologram scale can be regarded as a diffraction grating. The state of diffraction by the diffraction grating (hologram scale) is illustrated in FIG. 9. Generally, a diffraction grating generates diffracted lights of plural orders, such as 0-order diffracted light, xc2x11-order diffracted lights, and the like. In displacement detection, the interference of the xc2x11-order diffracted lights is utilized. P is the pitch of the diffraction grating, i.e., the graduation pitch of the hologram scale.
In order to understand the principles of detection, with reference to FIG. 10, we consider interference of +M-order diffracted light and +N order diffracted light of rays incident on the diffraction grating at xcex80. xcex8N is the angle of diffraction of the +N-order diffracted light, and xcex8M is the angle of diffraction of the +M-order diffracted light. The relationships between the angles of diffraction and the angles of incidence xcex80 are expressed by the following formulas, wherein xcex is the wavelength of the light source.                                           sin            ⁢                          xe2x80x83                        ⁢                          θ              N                                -                      sin            ⁢                          xe2x80x83                        ⁢                          θ              0                                      =                              N            ⁢                          xe2x80x83                        ⁢            λ                    P                                                              sin            ⁢                          xe2x80x83                        ⁢                          θ              M                                -                      sin            ⁢                          xe2x80x83                        ⁢                          θ              0                                      =                              M            ⁢                          xe2x80x83                        ⁢            λ                    P                    
Further, the intensity of the light I at point Q when the amplitudes of the respective interfering light waves are the same is expressed by the following formula.   I  =      2    ⁢          A      2        ⁢          {              1        +                  cos          ⁢                                                    2                ⁢                π                            λ                        ⁡                          [                              Δ                +                                                                                                    (                                                  N                          -                          M                                                )                                            ⁢                      λ                                        P                                    ⁢                  x                                            ]                                          }      
This formula shows that, when the diffraction grating is moved in the X direction or the xe2x88x92x direction, the light intensity I at point Q varies sinusoidally at the period x=P/(Nxe2x88x92M). The detecting optical system of the hologram encoder utilizes this phenomenon. For example, in a detecting optical system which makes the xc2x11-order diffracted lights interfere, when the scale is moved at a pitch of 1 graduation, the intensity of the light varies completely sinusoidally by two periods. Namely, in this case, the graduation pitch P of the scale is optically divided into two, and the basic resolution becomes twice as large. Further, by electrically dividing the electric signal, which is obtained by photoelectrically converting the change in the intensity of this light, by an interpolation circuit at a latter stage, a resolution which is greater than or equal to the basic resolution can be obtained.
Details of conventional hologram encoders are disclosed in, for example, xe2x80x9cUltra Precision Opto-Electrical Linear Encoder with Hologram Scalexe2x80x9d, by Masaki Tomiya and Motohiro Osaki in xe2x80x9cHikari Gijyutsu Kontakutoxe2x80x9d (xe2x80x9cOptical and Electro-Optical Engineering Contactxe2x80x9d), Vol. 38, No. 6 (2000), pp. 368-376.
However, the above-described conventional hologram encoder has the serious problem that there is much noise of the detection signal. The cause thereof is that, in addition to the xc2x11-order diffracted lights contributing to the interference, undesirable diffracted light (e.g., 0-order diffracted light or higher-order diffracted light of an order of 2 or more) is generated by the hologram scale. A slit for detecting the interference pattern of the xc2x11-order diffracted lights is provided in a photo-detector, but the 0-order diffracted light or higher-order diffracted light also is passed through the slit and is simultaneously incident on the photo-detector. This is a cause of noise. If the noise becomes great, it is difficult to detect the changes in the amplitude of the aforementioned sinusoidal signal, and the sensitivity of the system with respect to positional displacement deteriorates. As a result, the resolution of the displacement measurement deteriorates.
The present invention was developed in consideration of the above-described circumstances, and an object of the present invention is to provide an optical encoder and a scale for an encoder which can reduce noise of a detection signal and can detect the amount of movement of a scale at a high resolution.
In order to achieve the above object, the optical encoder of the present invention comprises: an optical sensor section in which a scale and a detecting optical system are disposed so as to be one of relatively movable and relatively rotatable, and an optically anisotropic region, which diffracts incident laser light and selectively rotates a polarization direction of diffracted light of a predetermined order in a predetermined polarization direction, is formed at the scale, and the detecting optical system includes a light source irradiating laser light onto the scale, a polarized light separating means separating a polarized light component of the predetermined polarization direction from diffracted light which has been one of transmitted through the scale and reflected by the scale, and a light intensity detecting means detecting light intensity at a predetermined position of an interference pattern due to the polarized light component which has been separated; and movement amount computing means for computing an amount of movement of the scale on the basis of a change in the light intensity detected at the optical sensor section.
In the optical encoder, in the detecting optical system, the laser light irradiated onto the scale from the light source is diffracted by the optically anisotropic region formed at the scale. At this time, the polarization direction of diffracted light of a predetermined order is selectively rotated in a predetermined polarization direction. The polarized light separating means separates a polarized light component of a predetermined polarization direction, i.e., diffracted light of a predetermined order, from the diffracted light which has been transmitted through the scale or reflected at the scale. The light intensity detecting means detects the light intensity at a predetermined position of the interference pattern due to the diffracted light which has been separated.
In the optical sensor section, the detecting optical system and the scale are disposed so as to be relatively movable or relatively rotatable. When the detecting optical system and the scale move relative to one another or rotate relative to one another, the diffraction interference pattern changes, and the light intensity detected at the light intensity detecting means changes. Then, the movement amount computing means computes the amount of movement of the scale on the basis of the change in the light intensity detected at the optical sensor section. For example, when the light intensity detected at the light intensity detecting means changes periodically, the amount of movement of the scale can be computed by associating the period of the change in the light intensity with the diffraction pitch of the scale.
In this way, at the optical encoder of the present invention, when the laser light is diffracted by the optically anisotropic region formed at the scale, the polarization direction of diffracted light of a predetermined order is rotated. Only the interference light of the diffracted light, whose polarization direction has been rotated, is detected at the photo-detector. Thus, a detection signal in which noise is reduced can be obtained, and the amount of movement of the scale can be detected at a high resolution.
A surface-emitting laser is suitable as the light source of the optical encoder. When a surface-emitting laser is used as the light source, the spreading of the beam can be kept relatively low. Thus, because there is no need for a collimator lens or a condensing lens, the number of parts can be reduced, and the device can be made more compact.
A polarizer, such as a polarizing plate or a polarizing filter or the like which transmits a linearly polarized light component of a predetermined polarization direction from the incident light, can be used as the polarized light separating means.
The scale for an encoder used in the optical encoder has the feature that an optically anisotropic region, which diffracts incident laser light and selectively rotates a polarization direction of diffracted light of a predetermined order in a predetermined direction, is formed at the scale for an encoder.
The optically anisotropic region formed at the scale can be a region having photo-induced anisotropy which is induced by the irradiation of light. Photo-induced birefringence and photo-induced dichroism are types of photo-induced anisotropy. The optically anisotropic region may be formed by inducing birefringence by irradiating light onto one of a recording material containing a polymer compound (which may be polymer liquid crystal) having a photoisomerizing group in a side chain, and a recording material containing a polymer compound in which photoisomerizing molecules are dispersed.
The polymer compound preferably has a asobenzene structure, and more preferably has an aromatic hydrocarbon group in the main chain. The polyester expressed by following general formula (1) is particularly preferable. 
In general formula (1), X represents a cyano group, a methyl group, a methoxy group, or a nitro group; Y represents a bivalent coupling group such as an oxy group, a carbonyl group, or a sulfonyl group; and m represent integers from 2 to 18; and n represents an integer from 5 to 500.
The optically anisotropic region may be formed by, for example, recording a polarization hologram.